Magnetic tunnel junction sensor having a longitudinal bias layer in contact with a free layer

ABSTRACT

A magnetic tunnel junction (MTJ) sensor in which the free layer longitudinal biasing elements are coupled, without insulation, to the free layer outside of the MTJ stack to provide reliable non-shunting MTJ free layer stabilization without extremely thin dielectric layers. In one embodiment, hard magnetic (HM) layers are disposed in contact with the free layer outside of and separated from the MTJ stack active region by a thick dielectric layer. In another embodiment, antiferromagnetic (AFM) bias layers are disposed in contact with the free layer outside of and separated from the MTJ stack active region by a thick dielectric layer. In other embodiments, nonconductive HM layers are disposed either in contact with the free layer outside of the MTJ stack active region and/or in abutting contact with the MTJ stack active region.

REFERENCE TO RELATED APPLICATION

This is a divisional application of application Ser. No. 09/848,674,filed May 3, 2001 now U.S. Pat. No. 6,833,982.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to a magnetic tunnel junction (MTJ)device and more particularly to an MTJ device for use as amagnetoresistive (MR) head for reading magnetically-recorded data.

2. Description of the Related Art

Computers often include auxiliary memory storage devices having media onwhich data can be written and from which data can be read for later use.A direct access storage device (DASD or disk drive) incorporatingrotating magnetic disks is commonly used for storing data in magneticform on the disk surfaces. Data is recorded on concentric, radiallyspaced tracks on the disk surfaces. Magnetic heads including readsensors are then used to read data from the tracks on the disk surfaces.

In high capacity disk drives, magnetoresistive (MR) read sensors (MRheads) are preferred in the art because of their capability to read dataat greater track and linear densities than earlier thin film inductiveheads. An MR sensor detects the magnetic data on a disk surface througha change in the MR sensing layer resistance responsive to changes in themagnetic flux sensed by the MR layer.

The early MR sensors rely on the anisotropic magnetoresistive (AMR)effect in which an MR element resistance varies as the square of thecosine of the angle between the magnetic moment of the MR element andthe direction of sense current flowing through the MR element. Recordeddata can be read from a magnetic medium because the external magneticfield from the recorded magnetic medium (the signal field) changes themoment direction in the MR element, thereby changing the MR elementresistance and the sense current or voltage.

The later giant magnetoresistance (GMR) sensor relies on thespin-scattering effect. In GMR sensors, the resistance of the GMR stackvaries as a function of the spin-dependent transmission of theconduction electrons between two magnetic layers separated by anon-magnetic spacer layer and the accompanying spin-dependent scatteringthat occurs at the interface of the magnetic and non-magnetic layers andwithin the magnetic layers. GMR sensors using only two layers offerromagnetic (FM) material separated by a layer of non-magneticconductive material (e.g., copper) are generally referred to as spinvalve (SV) sensors.

In 1995, a new class of high magnetoresistive (MR) materials wasdiscovered in which the nonmagnetic layer separating the two FM layersis made with an ultrathin nonconductive material, such as an aluminumoxide layer <20 Å thick. With the switching of magnetization of the twomagnetic layers between parallel and antiparallel states, thedifferences in the tunneling coefficient of the junction and thus themagnetoresistance ratio have been demonstrated to be more than 25%. Adistinctive feature of this magnetic tunnel junction (MTJ) class ofmaterials is its high impedance (>100 kΩ-μm²), which allows for largesignal outputs.

A MTJ device has two ferromagnetic (FM) layers separated by a thininsulating tunnel barrier layer. MTJ operation relies on thespin-polarized electron tunneling phenomenon known in the art. One ofthe two FM layers (the reference layer) has a higher saturation field inone direction because of, for example, a higher coercivity, than theother FM layer (the sensing layer), which is more free to rotate inresponse to external fields. The insulating tunnel barrier layer is thinenough so that quantum mechanical tunneling occurs between the two FMlayers. The tunneling phenomenon is electron-spin dependent, making themagnetic response of the MTJ a function of the relative momentorientations and spin polarizations of the two FM layers.

When used as memory cells, the MTJ memory cell state is determined bymeasuring the cell resistance to a sense current passed perpendicularlythrough the MTJ from one FM layer to the other. The charge carrierprobability of tunneling across the insulating tunnel barrier layerdepends on the relative alignment of the magnetic moments (magnetizationdirections) of the two FM layers. The tunneling current is spinpolarized, which means that the electrical current passing from one ofthe FM layers, for example, the reference layer whose magnetic moment ispinned to prevent rotation, is predominantly composed of electrons ofone spin type (spin up or spin down, depending on the referenceorientation of the magnetic moment). The degree of spin polarization ofthe tunneling current is determined by the electronic band structure ofthe magnetic material composing the FM layer at the interface of the FMlayer with the tunnel barrier layer. The FM reference layer thus acts asa spin filter for tunneling electrons. The probability of tunneling ofthe charge carriers depends on the availability of electronic states ofthe same spin polarization as the spin polarization of the electricalcurrent in the FM sensing layer. When the magnetic moment of the FMsensing layer is parallel to that of the FM reference layer, moreelectronic states are available than when the two FM layer magneticmoments are antiparallel. Accordingly, charge carrier tunnelingprobability is highest when the magnetic moments of both layers areparallel and is lowest when the magnetic moments are antiparallel.Between these two extremes, the tunneling probability assumes someintermediate value, so that the electrical resistance of the MTJ memorycell depends on both the sense current spin polarization and theelectronic states in both FM layers. As a result, the two orthogonalmoment directions available in the free FM sensing layer together definetwo possible bit states (0 or 1) for the MTJ memory cell. Seriousinterest in the MTJ memory cell has lagged for some time because ofdifficulties in achieving useful responses in practical structures atnoncryogenic temperatures.

The magnetoresistive (MR) sensor known in the art detects magnetic fieldsignals through the resistance changes of a read element, fabricated ofa magnetic material, as a function of the strength and direction ofmagnetic flux sensed by the read element. The conventional MR sensor,such as that used as a MR read head for reading data in magneticrecording disk drives, operates on the basis of the anisotropicmagnetoresistive (AMR) effect of the bulk magnetic material, which istypically permalloy (Ni₈₁Fe₁₉). A component of the read elementresistance varies as the square of the cosine of the angle between themagnetization direction in the read element and the direction of sensecurrent through the read element. Recorded data can be read from amagnetic medium, such as the disk in a disk drive, because the externalmagnetic field from the recorded magnetic medium (the signal field)causes a change in the direction of magnetization in the read element,which in turn causes a change in resistance of the read element and acorresponding change in the sensed current or voltage.

The use of an MTJ device as a MR read head is also well-known in theart. One of the problems with the MTJ read head is the difficultyencountered in developing a sensor structure that generates an outputsignal that is both stable and linear with respect to the magnetic fieldstrength sensed in the recorded medium. Some means is required tostabilize the magnetic domain state of the MTJ free FM sensing layer toprevent unacceptable Barkhausen noise arising from shifting magneticdomain walls within the free sensing layer. Also, some means forachieving a substantially linear response of the head is necessary foracceptable sensitivity. The longitudinal stabilization problem isparticularly difficult in an MTJ MR read head because, unlike an AMRsensor, the MTJ sense current passes perpendicularly through the stackof FM and tunnel barrier layers so that any metallic materials in directcontact with the edges of the FM layers act to shunt (short-circuit) theread head sense resistance.

Practitioners have proposed several methods for resolving these problemsto permit the use of MTJ sensors in magnetic read head applications. Forexample, in U.S. Pat. No. 5,729,410 (and later, in U.S. Pat. No.6,005,753), Fontana, Jr. et al. describe a MTJ device where the sensing(free) FM layer magnetic moment is longitudinally biased by a layer ofhard FM material located near but separated slightly from the side edgesthereof (and later, from the back edge thereof for added transversebiasing) by an intervening layer of electrically insulating material.The insulating layer isolates the hard biasing material from theelectrical leads and the sensing FM layer to prevent shunting of thesense current to the hard biasing material without interfering with theperpendicular sense current flow through the layers in the stack.Similarly, in U.S. Pat. No. 6,097,579, Gill proposes sandwiching apermanent magnet layer between two thin dielectric layers to providelongitudinal baising of the MTJ free layer. However, the Fontana, Jr, etal. and the Gill approaches are problematic to manufacture because theygenerally rely on extremely thin insulation layers to allow sufficientmagnetostatic coupling to reduce Barkhausen noise in the free FM layerwithout shunting the sense current.

FIG. 1A shows an illustrative embodiment of a magnetic tunnel junction(MTJ) sensor 10 from the prior art. Sensor 10 is viewed from the airbearing surface (ABS) so that, in operation, the magnetic medium (notshown) moves in the image plane vertically with respect to MTJ sensor10. MTJ sensor 10 includes an MTJ stack 12 disposed between a firstshield (S1) layer 14 and a second shield (S2) layer 16. MTJ stack 12 maybe characterized as an upper electrode 18 separated from a lowerelectrode 20 by a tunnel barrier 22. Upper electrode 18 includes aferromagnetic (FM) pinned layer 24 having a magnetic moment that ispinned by an exchange-coupled antiferromagnetic (AFM) layer 26, and asecond lead (L2) layer 28. The lower electrode 20 includes a FM freelayer 30 and a first lead (L1) layer 32. MTJ stack 12 operates in theusual manner known in the art except that the stabilization biasing offree layer 30 is provided by a hard magnetic (HM) layer 34 disposed oneach side of MTJ stack 12. To prevent a loss of sensitivity fromundesired sense current shunting, HM layers 34 are sandwiched betweentwo insulating layers 36 and 38 substantially as shown. Practitioners inthe art can readily appreciate that the several layers outside of MTJstack 12 should be precisely created in a series of steps following aninitial etching procedure. The usual processes known in the art giverise to misalignment between the narrow ends of the various layers atthe edges of MTJ stack 12, leading to unit performance variations andhigh unit rejection rates.

FIG. 1B shows an air bearing surface (ABS) view of another illustrativeembodiment of a MTJ sensor 40 from the prior art. MTJ sensor 40 can beconsidered to include the end regions 42 and 44 separated from eachother by a central region 46. The active region of MTJ sensor 40 is theMTJ stack 48 formed in the central region 46. MTJ stack 48 has agenerally rectangular shape with a front face (shown) at the ABS, a backedge (not shown) opposite to the front edge and two opposite side edges50 and 52. MTJ stack 40 includes a first electrode 54 and a secondelectrode 56 between which is disposed a tunnel barrier layer 58. Firstelectrode 54 includes a pinned layer 60, an AFM layer 62 and a seedlayer 64, where pinned layer 60 is disposed between tunnel barrier layer58 and AFM layer 62, which is disposed between pinned layer 60 and seedlayer 64. Second electrode 56 includes a free layer 66 and a cap layer68, where free layer 66 is disposed between tunnel barrier layer 58 andcap layer 68. AFM layer 62 is exchange coupled to pinned layer 60providing an exchange field to pin the magnetization direction of pinnedlayer 60 perpendicular to the ABS. The magnetization of free layer 66 isoriented parallel to the ABS (absent other external magnetic fields) andis free to rotate in the presence of a signal magnetic field. As withMTJ sensor 10 (FIG. 1A), free layer stabilization bias is provided bythe HB layers 70, which are sandwiched between the insulation layers 72and 74 to prevent sensitivity losses through shunting of MTJ stack 48.

In U.S. Pat. No. 5,930,087, Brug et al. disclose a flux-guide MTJ sensorhaving a FM free layer that extends beyond the active region (MTJ stack)to the sides and also to the rear and the front (to the ABS). Theysuggest in passing that the longitudinal biasing layer placed on theflux guide adjacent to each side of a flux-guide MTJ stack may consistof antiferromagnetic materials such as terbium-iron or nickel-oxide (anonconductor), but Brug et al. appear to prefer using antiferromagnetic(AFM) manganese compounds or permanent magnetic layers and neitherconsider nor suggest specific solutions to the MTJ sense currentshunting problem arising from such MTJ sensor geometries.

There is accordingly a need in the MR sensor art for an effective MTJlongitudinal biasing technique that can be implemented using simpler,more reliable fabrication methods leading to higher yields and moreconsistent unit performance without the sensitivity loss arising fromsense current shunting. These unresolved problems and deficiencies areclearly felt in the art and are solved by this invention in the mannerdescribed below.

SUMMARY OF THE INVENTION

This invention solves magnetic tunnel junction (MTJ) longitudinalbiasing problem by coupling, without insulation, the biasing elements tothe free layer outside of the MTJ stack to avoid shunting the tunnelcurrent passing through the MTJ stack.

It is a purpose of this invention to provide reliable MTJ free layerstabilization without extremely thin dielectric layers to preventunwanted shunting of tunnel current.

In one aspect, the invention is a MTJ sensor in a magnetic read head ina magnetic read head having an air bearing surface (ABS), the MTJ sensorincluding a MTJ stack with an active region disposed at the ABS andhaving two opposite sides each disposed generally orthogonally to theABS, the MTJ stack including an antiferromagnetic (AFM) layer spanningthe active region, a pinned layer of ferromagnetic (FM) material incontact with the AFM layer, a free layer of FM material spanning theactive region and extending beyond each of the two opposite sidesthereof, and a tunnel junction layer of electrically nonconductivematerial disposed between the pinned layer and the free layer in theactive region, the MTJ sensor further including a longitudinal biaslayer formed on and in contact with the free layer outside of the activeregion for biasing the magnetic moment of the free layer insubstantially a predetermined direction in the absence of an externalmagnetic field.

In another aspect, the invention is a MTJ sensor in a magnetic read headhaving an ABS, the MTJ sensor including a MTJ stack with an activeregion disposed at the ABS and having two opposite sides each disposedgenerally orthogonally to the ABS, the MTJ stack including an AFM layerspanning the active region, a pinned layer of FM material in contactwith the AFM layer, a free layer of FM material spanning the activeregion, and a tunnel junction layer of electrically nonconductivematerial disposed between the pinned layer and the free layer in theactive region, the MTJ sensor further including a nonconductivelongitudinal bias layer formed outside of the active region and inabutting contact with the two opposite sides of the active region forbiasing the magnetic moment of the free layer in substantially apredetermined direction in the absence of an external magnetic field.

In yet another aspect, the invention is a direct access storage device(DASD) including a magnetic recording disk having at least one surfacefor storing magnetically recorded data, a magnetic read head having anABS disposed for reading the data from the magnetic recording disksurface, the magnetic read head including a MTJ sensor having a MTJstack with an active region disposed at the ABS and having two oppositesides each disposed generally orthogonally to the ABS, the MTJ stackincluding an AFM layer spanning the active region, a pinned layer of FMmaterial in contact with the AFM layer, a free layer of FM materialspanning the active region and extending beyond each of the two oppositesides thereof, and a tunnel junction layer of electrically nonconductivematerial disposed between the pinned layer and the free layer in theactive region, the MTJ sensor further including a longitudinal biaslayer formed on and in contact with the free layer outside of the activeregion for biasing the magnetic moment of the free layer insubstantially a predetermined direction in the absence of an externalmagnetic field, the DASD further including an actuator for moving themagnetic read head across the magnetic recording disk surface to accessthe data stored thereon, and a data channel having sense circuitrycoupled electrically to the MTJ sensor for detecting changes inresistance of the MTJ sensor caused by rotation of the magnetic momentof the free ferromagnetic layer relative to the fixed magnetic moment ofthe pinned layer responsive to magnetic fields representing the datastored on the magnet recording disk surface.

In another aspect, the invention is a DASD including a magneticrecording disk having at least one surface for storing magneticallyrecorded data, a magnetic read head having an ABS disposed for readingthe data from the magnetic recording disk surface, the magnetic readhead including a MTJ sensor having a MTJ stack with an active regiondisposed at the ABS and having two opposite sides each disposedgenerally orthogonally to the ABS, the MTJ stack including an AFM layerspanning the active region, a pinned layer of FM material in contactwith the AFM layer, a free layer of FM material spanning the activeregion, and a tunnel junction layer of electrically nonconductivematerial disposed between the pinned layer and the free layer in theactive region, the MTJ sensor further including a nonconductivelongitudinal bias layer formed outside of the active region and inabutting contact with the two opposite sides of the active region forbiasing the magnetic moment of the free layer in substantially apredetermined direction in the absence of an external magnetic field,the DASD further including an actuator for moving the magnetic read headacross the magnetic recording disk surface to access the data storedthereon, and a data channel having sense circuitry coupled electricallyto the MTJ sensor for detecting changes in resistance of the MTJ sensorcaused by rotation of the magnetic moment of the free ferromagneticlayer relative to the fixed magnetic moment of the pinned layerresponsive to magnetic fields representing the data stored on the magnetrecording disk surface.

In still another aspect, the invention is a method for fabricating amagnetic tunnel junction (MTJ) sensor for use in a magnetic read headhaving an air bearing surface (ABS), the method comprising the unorderedsteps of (a) forming a MTJ stack with an active region disposed at theABS and having two opposite sides each disposed generally orthogonallyto the ABS, including the unordered steps of (a.1) forming anantiferromagnetic (AFM) layer, (a.2) forming a pinned layer offerromagnetic (FM) material in contact with the AFM layer, (a.3) forminga free layer of FM material, (a.4) forming a tunnel junction layer ofelectrically nonconductive material disposed between the pinned layerand the free layer, and (a.5) removing all material outside of theactive region from the AFM layer, the pinned layer, and the tunneljunction layer to define the two opposite sides of the active region,and (b) forming a longitudinal bias layer outside of the active regionin contact with the free layer for biasing the magnetic moment of thefree layer in substantially a predetermined direction in the absence ofan external magnetic field.

The foregoing, together with other objects, features and advantages ofthis invention, can be better appreciated with reference to thefollowing specification, claims and the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of this invention, reference is nowmade to the following detailed description of the embodiments asillustrated in the accompanying drawings, in which like referencedesignations represent like features throughout the several views. Forconvenience, the following abbreviations are used in the drawings: S1 isa first shield; S2 is a second shield; L1 is a first lead; L2 is asecond lead; PL is a pinned layer; FL is a free layer; AF (oralternatively AFM) is antiferromagnetic material; I1 is a firstinsulation; I2 is a second insulation; I3 is a third insulation: HB ishard bias; HM is hard magnet; ABS is air bearing surface; P1 and P2 arefirst and second poles; G1 is an insulating gap layer; G3 is another gaplayer; TB is a tunnel barrier; and, C1 and C2 are conductor layers.

FIGS. 1A and 1B show schematic representations of exemplary magnetictunnel junction (MTJ) sensors from the prior art wherein thelongitudinal biasing layers are sandwiched between two dielectriclayers;

FIGS. 2A and 2B show schematic representations of a MTJ sensorembodiment of this invention wherein non-shunting free-layerstabilization is provided by non-conducting antiferromagnetic (AFM)layers;

FIGS. 3A and 3B show schematic representations of a MTJ sensorembodiment of this invention wherein non-shunting free-layerstabilization is provided by conductive hard magnetic (HM) layersseparated from the MTJ stack by thick insulation layers;

FIGS. 4A and 4B show schematic representations of a MTJ sensorembodiment of this invention wherein non-shunting free-layerstabilization is provided by conductive AFM layers separated from theMTJ stack by thick insulation layers;

FIG. 5 shows an unscaled schematic representation of a verticalcross-section view of an inductive write/MTJ read head suitable for usewith the MTJ sensor of this invention;

FIG. 6 shows a schematic representation of a direct access storagedevice (DASD) suitable for use with the MTJ sensor of this invention;

FIG. 7 shows a schematic representation of a MTJ sensor embodiment ofthis invention wherein non-shunting free-layer stabilization is providedby non-conducting HM layers abutting the free layer and MTJ stack; and

FIG. 8 shows a block diagram illustration of a flow chart illustratingan exemplary method of this invention for fabricating the MTJ sensor ofthis invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 2A and 2B show schematic representations of the air bearingsurface (ABS) of exemplary embodiments of the MTJ sensor of thisinvention wherein the free layer stabilization is provided bynon-conducting antiferromagnetic layers made of material such as nickeloxide or certain phases of iron oxide. The MTJ sensor 76 (FIG. 2A)includes a central active region 78 disposed between two side regions 80and 82. A first lead (L1) layer 84 is a substrate on which the FM freelayer 86 is first deposited, followed by the tunnel barrier layer 88,the FM pinned layer 90 and the first AFM layer 92, which isexchange-coupled to FM pinned layer 90 for the purposes of pinning themagnetic moment of FM pinned layer 90 to the desired direction. Thelayer stack thus produced is then etched to remove all materials in theend regions 80 and 82 down to FM free layer 86, a portion of which isleft in the side regions 80 and 82 substantially as shown. After theetching step, which defines active region 78 of MTJ sensor 76, thesecond AFM layer 94 is deposited in direct contact with the portions offree layer 86 remaining in side regions 80 and 82. The material used forAFM layer 94 should be electrically insulating (such as nickel oxide orcertain phases of iron oxide). If first AFM layer 92 should be orientedmagnetically orthogonally to the magnetic moment second AFM layer 94,the materials should be selected so that the blocking temperatures ofAFM layers 92 and 94 differ sufficiently to permit the AFM layer 94 tobe magnetically set without disturbing the earlier magnetic settings offirst AFM layer 92.

Finally, a second lead (L2) layer 96 is deposited to complete the MTJsensor 76. FM free layer 86 is stabilized by the exchange-coupling ofAFM layer 94 to the portions of FM free layer 86 in the end regions 80and 82. This is unexpectedly advantageous because the stabilizationfield is well-controlled using simple easily-repeatable depositiontechniques. There is no requirement for careful alignment of taperedlayer edges formed using liftoff deposition techniques, nor forsophisticated etching techniques. Because AFM layer 94 isnon-conducting, there is no loss of MTJ stack sensitivity throughunwanted sensor current shunting.

In contrast with the “top” embodiment of MTJ sensor 76 (FIG. 2A), FIG.2B shows a representative embodiment of a “bottom” MTJ sensor 98, whichis analogous to the “top” MTJ sensor 76. MTJ sensor 98 includes theactive region 100 and the side regions 102 and 104. The first lead (L1)layer 106 is used as a substrate for depositing a first AFM layer 108, aFM pinned layer 110, a tunnel barrier layer 112 and a FM free layer 114.At this point during the fabrication procedure, all material is etchedfrom the side regions 102 and 104 down to first lead (L1) layer 106 sothat no trace of first AFM layer 108 remains. The second AFM layer 116is then deposited up to the existing level of free layer 114. Additionalferromagnetic material is deposited over both active region 100 and sideregions 102 and 104 to complete FM free layer 114 substantially as shownin FIG. 2B. Finally, a second lead (L2) layer 118 is deposited tocomplete MTJ sensor stack 98. As before, the material used for secondAFM layer 116 should be non-conducting to avoid undesired shuntinglosses to MTJ sensor sensitivity. Also, as discussed above in connectionwith FIG. 2A, the materials selected for first and second AFM layers 108and 116 should be chosen to permit the processing of AFM layer 116without exceeding the blocking temperature of the pre-existing AFM layer108.

The sensor of this invention may also be embodied using conductingmaterials for stabilizing the free layer, as is now described. FIGS. 3Aand 3B show schematic representations of the ABS of exemplary MTJ sensorembodiments of this invention wherein the free layer stabilization isprovided by conductive hard magnetic (HM) layers separated from the MTJstack active region by thick insulation layers. In FIG. 3A, the MTJsensor 120 is shown with the active region 122 separating the two endregions 124 and 126. This is a “top” MTJ sensor configuration. The firstlead (L1) layer 128 is deposited followed by the FM free layer 130, thetunnel barrier layer 132, the FM pinned layer 134 and the first AFMlayer 136. At this point, all layer materials outside the active region122 are etched down to but not completely through FM free layer 130.Using masking and liftoff techniques known in the art, a hard magnetic(HM) layer 138 is deposited only within the outer portions of endregions 124 and 126, respectively. A non-conducting insulating layer 140is deposited to fill the remaining empty portions of end regions 124 upto the level of first AFM layer 136. Finally, a second lead (L2) layer142 is deposited to complete the device. MTJ sensor 120 permitsexcellent free layer stabilization because of the direct contact of asubstantial portion of HM layer 138 with FM free layer 130 in endregions 124 and 126 without contact with any part of active region 122,thereby avoiding any loss of sensitivity from undesired shunting ofsense current.

FIG. 3B illustrates the “bottom” MTJ sensor 144 equivalent to the “top”MTJ sensor 120 (FIG. 3A). MTJ sensor 144 includes the active region 146separating the two end regions 148 and 150. Beginning with the firstlead (L1) layer 152, the MTJ stack is built beginning with the first AFMlayer 154 followed by the FM pinned layer 156, the tunnel barrier layer158, and the FM free layer 160. These material layers are then etched inend regions 148 and 150 down to expose first lead (L1) layer 152 and theinsulating layer 162 is then deposited to fill end regions 148 and 150up to the top of free layer 160, after which additional ferromagneticmaterial is added to build up free layer 160 over active region 146 andend regions 148 and 150, substantially as shown. Thereafter, using acombination of masking and liftoff techniques known in the art, the HMlayer 164 is deposited in the outer portions of end regions 148 and 150in direct contact with free layer 160 and, finally, the second lead (L2)layer 165 is deposited to complete MTJ sensor 144.

FIGS. 4A and 4B show schematic representations of the ABS of exemplaryMTJ sensor embodiments of this invention wherein the free layerstabilization is provided using conductive AFM layers separated from theMTJ stack by thick insulation layers, similarly conceptually to thediscussion above in connection with FIGS. 3A and 3B. In FIG. 4A, the topMTJ sensor 166 includes the active region 168 disposed between the twoside regions 170 and 172. Operation and fabrication of MTJ sensor 166may be appreciated with reference to the above discussion of FIG. 3Aexcept that, instead of HM layer 138 (FIG. 3A), MTJ sensor 166 uses theconductive AFM layers 173 to provide stabilization of the FM free layer174. By permitting conductive as well as non-conductive materials to beconsidered for second AFM layer 173, a wider range of choices is madeavailable for resolving material conflicts between second AFM layer 173and the first AFM layer 176.

FIG. 4B shows the “bottom” MTJ sensor 178 having the active region 180disposed between the two end regions 182 and 184. Fabrication andoperation of MTJ sensor 178 may be understood with reference to theabove discussion of FIGS. 3A and 4A. In FIG. 4B, the second AFM layer186 is disposed directly in contact with the FM free layer 188 in theouter portions of end regions 182 and 184. Again, permitting a widerrange of materials to be used for longitudinal stabilization of freelayer 188 may resolve otherwise difficult material conflicts.

FIG. 7 shows a schematic representation of the ABS of an exemplary MTJsensor embodiment of this invention wherein the free-layer stabilizationis provided by non-conducting HM layers abutting the free layer and MTJstack. The MTJ sensor 226 includes the active region 228 disposedbetween the two side regions 230 and 232. Fabrication and operation ofMTJ sensor 226 may be understood with reference to the above discussionof FIGS. 2A and 2B except that the etching step is conducted completelythrough the FM free layer 234 into the first lead (L1) layer 236 so thatFM free layer 234 is truncated at the edge of the active region and doesnot extend into side regions 230-232. The tunnel barrier layer 238, theFM pinned layer 240, the AFM pinning layer 242 and the first portion ofthe second lead (L2) layer 244 are also truncated at the edge of activeregion 228 by the same etch process. A nonconductive HM layer 246 isthen deposited in side regions 230-232 using any useful masking andlift-off procedure so that HM layer 246 material fills side regions 230and 232 and abuts the edges of the MTJ stack layers 234, 238, 240 and242 to provide stabilization of FM free layer 234 substantially asshown. Finally, the etching mask is removed and the remainder of secondlead (L2) layer 244 is deposited to complete MTJ sensor 226. Because HMlayer 246 is nonconductive (for example, a nickel-oxide or bariumferrite compound), there is no shunting of sense current flow in activeregion 228 (the MTJ stack). No thin insulator layers are needed and onlyone lift-off step is required. Because MTJ sensor 226 is symmetricexcept for active region 228, it may be readily appreciated that thisdiscussion in connection with FIG. 7 applies to either a “top” or“bottom” MTJ sensor configuration.

FIG. 5 shows an unscaled schematic representation of a vertical crosssection view of an inductive write/read head suitable for use with theMTJ sensor of this invention. FIG. 5 is a cross-sectional schematic viewof a read/write head 190 which includes the MTJ read head portion 192and the magnetic write head portion 194. Head 190 is lapped to form anair bearing surface (ABS), which is spaced from the surface of arotating disk (FIG. 6) by an air bearing as is well-known in the art.The read head includes a MTJ sensor 196 sandwiched between the first andsecond conductor layers C1 and C2, which are, in turn, sandwichedbetween the first and second shield layers S1 and S2 so that the sensecurrent may be conducted to MTJ sensor 196 through shields S1 and S2,which are otherwise separated by the insulating gap layer G1. In adirect access storage device (DASD) of this invention (FIG. 6), MTJsensor 196 may be one of the preferred embodiments discussed above. Inhead 190, the write head 194 includes a coil layer C and insulationlayer I2, which are sandwiched between the two insulation layers I1 andI3, which are, in turn, sandwiched between first and second pole piecesP1 and P2. A gap layer G3 is sandwiched between the first and secondpole pieces P1 and P2 at their pole tips adjacent the ABS for providinga magnetic gap. During writing, signal current is conducted through coillayer C and flux is induced into the first and second pole layers P1, P2causing flux to fringe across the pole tips at the ABS. This fluxmagnetizes circular tracks on a rotating disk surface (FIG. 6) during awrite operation. During a read operation, the magnetized regions on therotating disk surface inject external magnetic flux into MTJ sensor 196of read head 192, thereby inducing changes in the sense currentconductivity of sensor 196. These sense current changes are detected bymeans of sense circuitry for detecting changes in electrical resistance(FIG. 6). The head 190 is denominated a “merged” head in which thesecond shield layer S2 of the read head is employed as a first polepiece P1 for the write head. In a “piggyback” head (not shown), thesecond shield layer S2 and the first pole piece P1 are embodied asseparate layers.

FIG. 6 shows a direct access storage device (DASD) (disk drive) 198embodying the present invention. As shown, at least one rotatablemagnetic disk 200 is supported on a spindle 202 and rotated by a diskdrive motor 204. The magnetic recording media on each disk is in theform of an annular pattern of concentric data tracks (not shown) on thesurface 210 of disk 200. At least one slider 206 is positioned on thedisk 200, each slider 206 supporting one or more magnetic read/writeheads 208, where head 208 incorporates the MTJ sensor of this inventionas described above in connection with FIG. 5. As the disks rotate,slider 206 is moved radially in and out over the disk surface 210 sothat heads 208 may access different portions of disk 200 where desireddata is recorded. Each slider 206 is attached to an actuator arm 212 bymeans of a suspension 214. Suspension 214 provides a slight spring forcethat biases slider 206 against disk surface 210. Each actuator arm 212is attached to an actuator 216. As shown in FIG. 3, actuator 216 may bea voice coil motor (VCM), for example. The VCM comprises a coil movablewithin a fixed magnetic field, the direction and speed of the coilmovements being controlled by the motor current signals supplied by acontrol unit 218.

During operation of DASD 198, the rotation of disk 200 generates an airbearing between slider 206 and the disk surface 210 that exerts anupward force or lift on the slider. The surface of slider 206, whichincludes head 208 and faces the surface of disk 200, is denominated anair bearing surface ABS). The air bearing thus counterbalances theslight spring force of suspension 214 and supports slider 206 off andslightly above disk surface 210 by a small, substantially constantspacing during normal operation. The various components of DASD 198 arecontrolled in operation by control signals generated by control unit218, such as access control signals and internal clock signals (notshown). Typically, control unit 218 includes logic control circuits,storage chips and a microprocessor (not shown). Control unit 218generates control signals to control various system operations such asdrive motor control signals on line 220 and head position and seekcontrol signals on line 222. The control signals on line 222 provide thedesired current profiles to optimally move and position slider 206 tothe desired data track on disk 200. Read and write signals arecommunicated to and from the read/write heads 208 by means of therecording channel 224, which includes sense circuitry for detectingchanges in electrical resistance in the MTJ sensor in head 208. Thisdescription of a typical DASD and the accompanying illustration of FIG.6 are for representational purposes only. It may be readily appreciatedby those skilled in the art that disk storage systems may contain alarge number of disks and actuators, and each actuator may support anumber of sliders, for example.

FIG. 8 shows a flow chart illustrating an exemplary method of thisinvention for fabricating the exemplary MTJ sensor 76 (FIG. 2A) of thisinvention. In the step 248, a first lead (L1) layer formed and serves asa substrate on which the FM free layer is deposited in step 250,followed by the conducting tunnel barrier layer in the step 252, the FMpinned layer in the step 254 and the first AFM layer in the step 256.Finally, a photoresist layer is added in the step 258 and treated toremove the resist over everything except the active region in the step260. The layer stack thus produced is then etched in the step 262 toremove all materials in the end regions down to (but not completelythrough) the FM free layer, a portion of which remains in the sideregions. After etching step 262, which defines the MTJ stack, thelongitudinal biasing layer is deposited in the step 264 directly overthe free layer portions exposed in the side regions. After the remainingphotoresist is washed away (together with the unwanted longitudinal biaslayer material in the active region) in the step 266, a second lead (L2)layer is deposited in the step 268 to complete the MTJ sensor.

From the above description of the MTJ sensor of this invention, is maybe readily appreciated by those skilled in the art that artificial AFMsubsystems such as FM/TM/FM sandwiches (where TM includes rubidium,rhenium, chromium and/or copper) may also be used to effectively enhancethe stiffness of the exchange biasing where appropriate. Moreover, itmay be readily appreciated by those skilled in the art that “currentperpendicular to plane” (CPP) giant magnetoresistive (GMR) sensors maybe fabricated in accordance with the teachings embodied in the exemplaryMTJ sensor embodiments of this invention discussed herein.

Clearly, other embodiments and modifications of this invention may occurreadily to those of ordinary skill in the art in view of theseteachings. Therefore, this invention is to be limited only by thefollowing claims, which include all such embodiments and modificationswhen viewed in conjunction with the above specification and accompanyingdrawing.

1. A magnetic read head having an air bearing surface (ABS), a magnetictunnel junction (MTJ) sensor for connection to sense circuitry fordetecting changes in electrical resistance within the sensor, the sensorcomprising: a MTJ stack with an active region disposed at the ABS andhaving two opposite sides each disposed generally orthogonally to theABS, the MTJ stack comprising: an antiferromagnetic (AFM) layer spanningthe active region, a pinned layer of ferromagnetic (FM) material incontact with the AFM layer, a free layer of FM material spanning theactive region and extending beyond each of the two opposite sidesthereof, and a tunnel junction layer of electrically nonconductivematerial disposed between the pinned layer and the free layer in theactive region; and a longitudinal bias layer formed on and in contactwith the free layer outside of the active region for biasing themagnetic moment of the free layer in substantially a predetermineddirection in the absence of an external magnetic field, wherein thelongitudinal bias layer comprises an electrically nonconductive AFMmaterial disposed outside of the active region and in abutting contactwith the two opposite sides of the active region.
 2. A direct accessstorage device (DASD) comprising: a magnetic recording disk having atleast one surface for storing magnetically recorded data; a magneticread head having an air bearing surface (ABS) disposed for reading thedata from the magnetic recording disk surface; in the magnetic readhead, a magnetic tunnel junction (MTJ) sensor comprising: a MTJ stackwith an active region disposed at the ABS and having two opposite sideseach disposed generally orthogonally to the ABS, the MTJ stackcomprising: an antiferromagnetic (AFM) layer spanning the active region,a pinned layer of ferromagnetic (FM) material in contact with the AFMlayer, a free layer of FM material spanning the active region andextending beyond each of the two opposite sides thereof, and a tunneljunction layer of electrically nonconductive material disposed betweenthe pinned layer and the free layer in the active region; and alongitudinal bias layer formed on and in contact with the free layeroutside of the active region for biasing the magnetic moment of the freelayer in substantially a predetermined direction in the absence of anexternal magnetic field; an actuator for moving the magnetic read headacross the magnetic recording disk surface to access the data storedthereon; and a data channel having sense circuitry coupled electricallyto the MTJ sensor for detecting changes in resistance of the MTJ sensorcaused by rotation of the magnetic moment of the free ferromagneticlayer relative to the fixed magnetic moment of the pinned layerresponsive to magnetic fields representing the data stored on the magnetrecording disk surface, wherein the longitudinal bias layer comprises anelectrically nonconductive AFM material disposed outside of the activeregion and in abutting contact with the two opposite sides of the activeregion.